1. Field of the Invention
The invention relates to an image forming apparatus in which a sheet on which an image was formed by an image forming section is again fed to the image forming section and images are formed onto the sheet a plurality of times.
2. Related Background Art
Hitherto, a copying apparatus which can form images/image onto both sides or one side of a copy transfer sheet has been put into practical use as a product and the conveyance of the transfer sheet is controlled by various kinds of conveying system.
For instance, in the conveyance control called a setback system, when a transfer sheet on which an image was formed on a conveying path provided on a copying apparatus main body is again fed to an image forming section through a lower conveying path in the apparatus, a plurality of transfer sheets on which images have already been formed and are enclosed onto the lower conveying path are stacked in a stake in which they are partially overlaid each other, thereby preventing that a plurality of transfer sheets are simultaneously conveyed when the transfer sheet is fed from the lower conveying path. Particularly, the setback system makes the most of the effect in the case where an arrangement space of the lower and upper conveying paths is limited in association with a demand to reduce the size of main body.
According to the conventional copying apparatus, an image forming apparatus in which a transfer sheet is conveyed by several rollers comprising different driving means upon formation of an image is constructed in a manner such that the several rollers comprising the different driving means sandwich the transfer sheet and even in case of driving from the stop state, the several rollers comprising the different driving means are simultaneously driven by driving commands of the same timing. When stopping, the driving operations of the different several rollers are also simultaneously stopped by drive stopping commands of the same timing.
In the format case, however, the same operations (namely, the operations such that the transfer sheet is run into the first setback roller on the front side in the conveying direction, a predetermined amount of loop is formed, and after that, the first setback roller is forwardly rotated, the second setback roller on the rear side in the conveying direction is also forwardly rotated, the transfer paper is forwardly moved by a predetermined amount, the first and second setback rollers are reversely rotated, and the transfer sheet is backwardly moved by a predetermined amount and stopped) are continuously repeated every transfer sheet, thereby executing the setback operation. However, in order to operate so as to accurately set a deviation amount of each sheet to a predetermined value, there are the following problems.
(1) After completion of the formation of one image of one surface, since the transfer sheet is conveyed along a long conveying path, the sheet is run into the first setback roller and a relatively large loop is formed and an oblique movement of the sheet must be prevented.
(2) When pressures of the first and second setback rollers to convey the transfer sheet are too large, the image on the first surface formed on the transfer sheet is rubbed with the rollers, the image becomes dirty, and the back side of the transfer sheet becomes dirty. Therefore, the roller pressures cannot be set to large values.
(3) There is a case where in a state in which the first setback roller sandwiches the formar transfer sheet, another transfer sheet runs into the first setback roller and is interposed between the first setback roller and the former transfer sheet upon formation of a loop. Thus, a stacking state is broken and the deviation amount of each sheet which should be constant varies.
(4) In dependence of the number of transfer sheets sandwiches by the first setback roller, a difference occurs in a thrust amount between the first setback roller and the stacked transfer sheet upon formation of a loop, so that the transfer sheet which will be stacked later is stacked in a state in which a deviation amount of each sheet is not constant.
Therefore, when the transfer sheet is again fed from the lower conveying unit to form an image of the second surface, a defective separation of each transfer sheet occurs and a sheet jam occurs.
In the latter case, on the other hand, several rollers comprising the different driving means sandwich the transfer paper and in case of driving from the stop state, the apparatus operates so as to simultaneously drive different several rollers by the driving commands of the same timing. Therefore, there are the following problems.
(1) In the case where a rise time of the driving of the rear side roller is faster than that of the front side roller for the moving direction of the transfer sheet, a loop of the transfer sheet is formed between the front side roller and the rear side roller. Consequently, the transfer sheet is subjected to a damage such as bending, waving, or the like. Further, when the image is copy transferred to the sheet, an image blank portion occurs on the sheet.
(2) In the case where the rise time of the driving of the rear side roller is slower than that of the front side roller for the moving direction of the transfer sheet, the transfer sheet is stretched between the front side roller and the rear side roller, so that the sheet is obliquely moved or a defective conveyance is caused.
(3) In the case where the transfer sheet onto which images have already been copy transferred in the both-sides/multiplex mode or the like, the images on the transfer sheet are rubbed by the rear side roller, so that the images and roller become dirty.
(4) Even in case of stopping the transfer sheet from the drive conveying state, there are various problems such that the transfer sheet is rubbed, the images and roller become dirty, and the like because of an asynchronizing state between both of the roller drive timings.